Currently, the 3rd Generation Partnership Project (3 GPP) is on the way in the long term evolution of 3 G mobile communication system and has determined to adopt the Orthogonal Frequency Division Multiplexing (OFDM) technology as the basic air interface technology. According to the OFDM technology, wide band of the system is divided into several narrow sub frequency bands. For example, if a system transmits data with a bandwidth of 20 MHz, the bandwidth can be divided into several to tens sub frequency bands, wherein each sub frequency band is of tens KHz to hundreds KHz and data are transmitted on each sub frequency band.
The Orthogonal Frequency Division Multiple Access (OFDMA) technology is a kind of multiple access technology which combines the OFDM technology with the Time Division Multiple Access (TDMA) technology and the Frequency Division Multiple Access (FDMA) technology, i.e. users are distinguished according to time and frequency.
In the communication system based on OFDMA, scheduling based on the frequency domain is a method which can effectively improve the communication performance. The main principle is that in the range of the whole system bandwidth, different users can select an optimal sub frequency band to transmit data according to their channel environment so that to realize the multi-user diversity. In order to perform the scheduling based on the frequency domain, the user needs to receive the downlink common pilot covering the whole system bandwidth, measure the channel quality of each sub frequency bandwidth, feed back the measurements to the base station and the base station performs the scheduling based on the frequency domain according to the measurements. It can be seen that the scheduling based on the frequency domain needs at least one transmission on the downlink and a subsequent transmission on the uplink, which is one scheduling time period.
In the TDD system based on the OFDMA, in order to realize the scheduling based on the frequency domain, i.e. the user may choose an optimal sub frequency band from the whole frequency band to perform the data transmission, a common pilot symbol should be transmitted in the downlink slot so that the user may perform the selection of the optimal sub frequency band.
Currently, in the long term 3 G evolution system, for the FDD (Frequency Division Duplex) system adopting the OFDMA based on the OFDM modulation, it uses a frame structure of 10 ms. As shown in FIG. 1, one frame includes 20 0.5 ms sub-frames (or slots), wherein each sub-frame (or slot) includes one downlink common pilot. The user may choose the optimal sub frequency band to transmit data according to these downlink common pilots.
However, the method of including a downlink common pilot in each slot is not adaptable to the TDD mode because for the TDD mode, each slot can both be used for the downlink and the uplink and only during the downlink transmission the downlink common pilot needs to be inserted; if several continuous slots are used for downlink transmission, the downlink common pilot needs to be inserted in only one downlink slot. Since there is a lack of the necessary uplink feedback, if the common pilot is inserted in each downlink slot, there would be too much system overhead.